Encapsulation of Disease-Causing and Commensal Mitis Group Non-Pneumococcal Streptococci

Background: Mitis group non-pneumococcal streptococci (MGNPS), specifically Streptococcus mitis, Streptococcus infantis and Streptococcus oralis, have recently been shown to cause pneumonia and/or bacteremia. These organisms often have capsular (cps) operons resembling those in pneumococci, and some express cps-generated polysaccharides that antigenically cross-react with pneumococcal capsular serotypes. But, to date, a series of MGNPS isolates has not been studied by electron microscopy (EM) for the presence of a capsule. Methods: We studied 21 MGNPS; 11 were isolated from sputum and determined to have caused pneumonia, 3 were isolated from blood, and 7 were commensal isolates cultured from the oral cavity of healthy adults. Two reacted with a pneumococcal anticapsular antibody. Isolates were fixed with two different protocols and examined by transmission EM. Results: EM of MGNPS after standard fixation and staining with uranyl acetate did not show capsules. In contrast, the 21 MGNPS isolates that we studied after fixation with ruthenium red and lysine acetate were all shown to be encapsulated. The thickness and density of capsules was related to their species: Streptococcus pneumoniae had the most prominent encapsulation and Streptococcus oralis had the least. However, within a species, there was no apparent difference in capsules between disease-causing and colonizing strains. Conclusions: EM with ruthenium red staining demonstrated capsules on 21 MGNPS, but within a species, there was no apparent difference between disease-causing and commensal isolates. It seems reasonable to conclude that the capsule, together with inoculum size, host's ability to clear aspirated organisms, and other as yet unidentified virulence factors, all contribute to the pathogenesis of MGNPS pneumonia.